The present invention relates generally to electronic devices and, more specifically, to a circuit for providing a redundant bond pad for probing semiconductor devices.
As seen in FIG. 1, one or more dies are formed in a conventional manner on a wafer which, in turn, is formed from a semiconductor material such as silicon. Each die has an integrated circuit or device that has been formed but not yet detached from the wafer. Further, each die on the wafer can be tested by placing a set of mechanical probes in physical contact with the die""s bond pads. The bond pads provide a connection point for testing the integrated circuitry formed on the die. The probes apply voltages to the input bond pads and measure the resulting output electrical signals on the output bond pads. Not all bond pads on a die, however, are easily accessible by these devices. Given the dies"" arrangement in FIG. 1, for example, it is generally easier to probe the long sides of the die; the short sides of the die are usually too close to the other dies to allow sufficient clearance for testing purposes. Thus, it can be difficult to test circuits that are coupled to an inaccessible bond pad.
Requiring bond pads to be located only in the areas accessible during testing may lead to inefficient and complex circuit layouts. One known solution, as shown in FIG. 3, is to attach another bond pad, one that can be reached by a testing device, to the same wire used by the original bond pad. This solution, however, tends to increase the input capacitance. Attempts at minimizing this capacitance will result in the use of more die space.
A second known solution is to multiplex (mux) two input buffers together, as illustrated in FIG. 4, once again allowing an testable bond pad to access circuitry. With this mux circuit, however, signals from the original pad take longer to reach the die""s integrated circuitry. In addition, if input is designed to be received from multiple input buffers in a parallel configuration, this muxing solution would require duplicating large portions of the input circuitry, once again taking up a great deal of die space.
The present invention provides a circuit allowing an alternate access point to be used in testing the integrated circuitry, wherein the circuitry is usually accessed at another point that is difficult to reach with testing equipment. The resulting advantage of this implementation is that the circuit may be easily tested. As another advantage, the circuit may operate during testing at the same polarity input as used in normal operations of the die without an increase in capacitance. Moreover, the preferred embodiments of this invention may be used to test the circuit without appreciably slowing down the time to input signals. Further, the invention will not require the duplication of circuitry related to the input of data. For purposes of testing in one preferred implementation, the circuit also prevents the use of an input pad employed during normal operation.